If you are using a memory card, take no more pics until you've recovered the photos, take the card out of the camera. This is to reduce the chance of you overwriting some of your "lost" photos. There are many good freeware photo recovery software options that are available to help you recover your photos. This blog article
reviews and outlines several of the better ones.
All the download links listed in the article are through CNET, primarily to verify that these are malware free software, and to give you a chance to read user reviews on the software before downloading.